Artistic impression of a superconducting quantum heat engine.
Credit
Heikka Valja / Aalto University
The future of quantum computers: Researchers at Aalto University have successfully built the world’s first cyclic quantum heat engine inside a superconducting circuit
The nanoscale device merges the laws of classical thermodynamics with quantum mechanics, using the microscopic heat found in ultracold environments to produce useful energy.
The study, published in Nature Communications, introduces a hardware architecture that could vastly reduce the physical complexity and cost of scaling up high-qubit quantum computers.
Scaling down the Otto cycle to the quantum realm
In classical physics, a heat engine (like a car’s internal combustion engine or James Watt’s steam engine) converts thermal energy into mechanical work by moving heat between a hot source and a cold sink.
The Aalto University team, led by Academy Professor Mikko Möttönen, recreated this exact thermodynamic process, specifically an Otto cycle, on a subatomic scale. However, instead of pistons, valves, and burning fuel, the quantum heat engine relies on three microscopic components fabricated onto a superconducting chip:
The transmon qubit:
One of the standard building blocks of modern quantum computing, serving as the central working medium of the engine.
A resonator:
Used to capture and monitor the energy states.
A quantum-circuit refrigerator:
A highly specialised, tunable component that replaces external hot and cold reservoirs.
Operated inside a cryostat at temperatures just above absolute zero, the single-quantum refrigerator is controlled via automated microwave pulses. By dynamically adjusting the control pulses, scientists can command the refrigerator to alternate between heating and cooling the transmon qubit on demand.
As the qubit is cycled through these thermal states, it undergoes quantum superposition and tunnelling, successfully generating a measurable output of positive work.
Eliminating the million-euro cable bottleneck
While demonstrating a cyclic quantum heat engine solves fundamental questions about how thermodynamics operates at the quantum scale, its primary value lies in solving a massive engineering bottleneck threatening the future of quantum computing.
Current national quantum strategies aim to scale systems up to thousands of logical qubits over the next decade, which requires hundreds of thousands of physical qubits. Under current hardware constraints, managing that many qubits presents a severe physical challenge:
High costs:
Operating a massive qubit array requires millions of specialised coaxial microwave cables running from room-temperature control systems down into the sub-zero cryostat. These cables cost roughly one thousand euros each.
Signal noise:
Packing millions of physical cables into a tight space leaks heat and introduces electronic noise, destabilising the fragile quantum states (coherence) of the computer.
The autonomous future
To eliminate this mess of wiring, the Aalto research team is optimising their device to create an autonomous quantum heat engine.
An autonomous version of this circuit would be capable of performing vital computing tasks—such as reading out the data states of neighbouring qubits—on-chip, using its own internal thermal cycles. By managing data processing locally at millikelvin temperatures, the engine eliminates the need to route microwave pulses back and forth to room-temperature equipment.
This internal automation could erase the need for millions of control cables, drastically lowering the cost, noise, and structural complexity of high-qubit quantum supercomputers.